The present invention relates to image-transfer devices and, more particularly, to devices, such as scanners and printers, that transfer images between digital and hard-copy formats.
Much of modern progress is associated with the increasing prevalence of computers that manipulate data in digital form. Peripherals such as scanners and printers provide important links between the human and computer realms. In addition, related technologies, such as facsimile (fax) machines and digital copiers are becoming evermore prevalent.
Most image-transfer devices, excluding some flat-bed scanners, require some form of sheet-feed device, either for handling documents to be scanned or for handling blank media to which an image is to be transferred. (As used herein, “blank media” includes all media to which images are to be added, even media with pre-existing images, such as letterhead stationery.) Many scanners use automated document feeders. Printers typically include a blank-media sheet feeder. In fact, many printers have mechanisms that support two or more media sources. Devices that support both printing and scanning, such as fax machines and multifunction office machines, typically use both document feeders and blank-media feeders.
As the costs of electronic and optical components are falling, sheet-feed mechanisms are consuming increasing proportions of the costs of image-transfer devices. The market expectation is that the sheet-feeding mechanisms draw media straight (i.e., “true”) through the imaging transfer section of a device. In many cases, the true media feed is to be accomplished with a wide variety of sheet media, including paper of different weights, card stock, transparency film, and photo-paper. Moreover, sheet feeding must be performed to specifications over thousands of sheets.
Deviations from straight paper feeding can result in image skew, i.e., the image is tilted relative to the media. In the case of scanning, the digital image is tilted relative to the sheet-media source. In the case of printing, the sheet-media image is tilted relative to the digital image source. In general, perceptible skew is highly undesirable, although unperceptible skew may be tolerable for some applications. However, manufacturing a sheet-feed mechanism that maintains skew within acceptable tolerances for different media over thousands of feeds is costly.
There is scanning software that can be used to correct for skew in an image after it has been completely transferred to a host computer. Such software makes assumptions about what an image should look like, e.g., the software assumes lines should be either horizontal or vertical, rather than oblique; such software may not be effective for images that do not conform to those assumptions. Furthermore, such software only applies to scanning, not to printing. What is needed is a more economical method for maintaining printed and scanned image skew within acceptable tolerances regardless of the type of image.